My recommendation would be to go to car-parts.com, and enter the year you're looking for; I did that when I was searching for spindles, and found a LOT of entries, with some as low as $40 including drums (I was looking for drum spindles).

Alternatively, you can search for just about any year of Granada or Monarch spindles, and those will fit early Mustangs as well.

I have a set of granada spindles w/ 11" rotors if you are just looking for front disc brakes, if you are looking for the original stuff though, well it turns out I dont have it(even though I thought I did before installing my mustang II suspension)